Slew rate is defined to be the maximum rate of change in output voltage. This can be measured using a function generator and oscilloscope, and is measured in Volts per micro second (V/μs). The higher the slew rate is, the faster the output can change. Since Slew Rate imposes high frequency limitations on the device it is beneficial to find methods to improve slew rate.
One method to improve slew rate is adding resistors to each source of transistors which take plus or minus input signal at their gate in differential input stage. This improvement is achieved thanks to the fact that, by adding the resistors, open loop gain is reduced and output resistance of the input stage is enhanced. So phase compensation capacitor (Cc) required for stability of the circuit can be reduced, and the reduced Cc leads to improvement of slew rate. However, as mentioned above, this improvement is achieved at expense of open loop gain of the operational amplifier. With this reduced open loop gain, several properties like input and output impedance get worsened.
In order to improve slew rate without worsening properties, certain modifications can be done to the circuit. The objective of the present invention is to enhance the impedance of the gain stage and reduce the compensation capacitance without sacrifice of open loop gain.